<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .a{
            height: 200px;
            background-color: #ed1250;
        }
        .b{
            height: 200px;
            background-color: #ed1250;
        }
        .blank{
            /*空元素自己外边距塌陷了*/
            margin-top: 10px;
            margin-bottom: 10px;
        }
    </style>
</head>
<body>
<div class="a">A</div>
<div class="blank"></div>
<div class="b">B</div>
</body>
</html>